1. Field of the Invention
The present invention relates generally to a scheduling apparatus and method for a communication system, and in particular, to an apparatus and method for performing scheduling using channel quality information (CQI) for only particular carriers in a communication system using multiple carriers (multicarrier communication system).
2. Description of the Related Art
Extensive research is being conducted on a 4th generation (4G) communication system which is the next generation communication system to provide users with services having various Qualities of Service (QoSs) supporting a high data rate. In particular, extensive research is being conducted on the 4G communication system to support a high-speed service guaranteeing mobility and a QoS in a Broadband Wireless Access (BWA) communication system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system.
In order to provide a high-speed, high-quality wireless multimedia service, the 4G communication system requires broadband spectrum resources. However, the use of the broadband spectrum resources increases a fading effect in a wireless transmission path due to multipath propagation, and causes a frequency selective fading effect even in a transmission band. Therefore, for the high-speed wireless multimedia service, an Orthogonal Frequency Division Multiplexing (OFDM) scheme which is robust against frequency selective fading is popularly used in the 4G communication systems.
The OFDM scheme, a scheme for transmitting data using multiple carriers, is a kind of a Multi-Carrier Modulation (MCM) scheme that prior to transmission parallel-converts a serial input symbol stream into parallel symbols and modulates the parallel symbols with a plurality of orthogonal subcarriers. The OFDM scheme has the advantage of being capable of maximizing throughput using a link adaptation scheme in addition to being robust against the frequency selective fading.
A multiple access scheme based on the OFDM scheme is referred to as an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. The OFDMA scheme reorders particular subcarriers among all of subcarriers into a subchannel and allocates the subchannel to a particular subscriber station (SS). The “subchannel” refers to a channel comprised of at least one subcarrier. The use of the OFDMA scheme enables a dynamic resource allocation in which a subchannel can be dynamically allocated to a particular SS based on a fading characteristic of a wireless channel, and in the OFDMA scheme, an increase in number of SSs, i.e. an increase in number of users, increases a ‘multiuser diversity gain’. Therefore, extensive studies of the OFDMA scheme is being conducted in the 4G communication system that requires relatively high throughput.
In all of the communication systems in which a multiuser environment is implemented, it is necessary to efficiently allocate resources to each user, i.e. SS, and a communication system using the OFDMA scheme (OFDMA communication system) should also efficiently allocate resources. For convenience, a description of a communication system supporting the multiuser environment (multiuser environment communication system) will be made herein with reference to the OFDMA communication system. A description will now be made of scheduling schemes proposed for the efficient allocation of resources in the multiuser environment communication system.
The scheduling scheme refers to a scheme for allocating resources to each SS, and the typical scheduling schemes include a Maximum Carrier-to-Interference ratio (Max C/I) scheme, a Maximum Fairness (MF) scheme, and a Proportional Fairness (PF) scheme. A description will now be made of the Max C/I scheme, the MF scheme, and the PF scheme.
(1) Max C/I Scheme
The Max C/I scheme is a scheduling scheme for allocating particular subcarriers to an SS having the best channel state from among a plurality of SSs in the multiuser environment communication system, i.e. the OFDMA communication system, and the use of the Max C/I scheme maximizes a multiuser diversity gain, thereby maximizing the entire throughput of the OFDMA communication system. Each of the SSs reports a state of a channel to its associated base station (BS), i.e. channel quality, for example, CQI representing a C/I, back to the BS, and the BS regards, as an SS having the best channel state, an SS that has fed back a CQI representing the maximum C/I among CQIs fed back from the SSs.
In the Max C/I scheme, if C/Is for all of the subcarriers have small values because of a very poor channel state of a particular SS, the particular SS cannot be allocated any subcarrier on a certain occasion. That is, because the Max C/I scheme allocates corresponding subcarriers to an SS having the best channel state for all of the subcarriers used in the OFDMA communication system, an SS having a very poor channel state may fail to be allocated even one subcarrier on a certain occasion.
The Max C/I scheme allocates subcarriers based only on a channel state between a BS and an SS, thus making it impossible to guarantee fairness between SSs. However, the Max C/I scheme which is easily implemented is superior to the other scheduling schemes in terms of complexity.
Due to the foregoing characteristics, the Max C/I scheme is chiefly used for the maximization of the entire throughput of the OFDMA communication system even though fairness between SSs is not guaranteed. However, the Max C/I scheme's advantage of maximizing the entire throughput of the OFDMA communication system is acquired only when each of the SSs can feed back CQIs for all subcarriers. That is, when each of the SSs feeds back CQIs for only particular subcarriers instead of all of the subcarriers, the Max C/I scheme's advantage of maximizing the entire throughput of the OFDMA communication system is not guaranteed. The reason for considering the case in which the SS feeds back CQIs for only particular subcarriers instead of all subcarriers in the OFDMA communication system is because the CQI feedback for all of the subcarriers causes an uplink load and functions as uplink interference to other SSs. Therefore, there is extensive research being conducted on feeding back CQIs for only particular subcarriers instead of all the subcarriers.
(2) MF Scheme
The MF scheme is a scheduling scheme proposed to maximize the throughput of an SS allocated the minimum throughput among throughputs of a plurality of SSs. Compared with the Max C/I scheme, the MF scheme can guarantee some lever of fairness between the SSs. Although the MF scheme actually maximizes the fairness between the SSs as compared with the Max C/I scheme, the entire throughput of the OFDMA communication system is undesirably reduced due to the fairness between the SSs.
Generally, the MF scheme compulsorily allocates a subcarrier for an SS having a very poor channel state, if any. Therefore, compared with allocation of the subcarrier to an SS having the best channel state, i.e. the maximum C/I, the allocation of the subcarrier to the SS having a very poor channel state causes a decrease in throughput, thereby reducing the entire throughput of the OFDMA communication system.
Due to the foregoing characteristics, the use of the MF scheme is not taken into consideration in a wireless communication system like the OFDMA communication system. Instead, the use of the MF scheme is chiefly used in a wire communication system in which a bottleneck link due to setup of many links between a source and a destination may fatally affect the entire performance of the communication system. In addition, the MF scheme is significantly higher than the Max C/I scheme in terms of implementation complexity. Actually, the use of the MF scheme is scarcely taken into consideration in the OFDMA communication system.
The MF scheme's advantage of maximizing fairness between SSs is acquired only when each of the SSs feeds back CQIs for all of the subcarriers. That is, when each of the SSs feeds back CQIs for only particular subcarriers instead of all of the subcarriers, the MF scheme's advantage of maximizing fairness between SSs is not guaranteed.
(3) PF Scheme
The PF scheme, a combined scheme of the Max C/I scheme and the MF scheme, is a scheduling scheme for maximizing the entire throughput while guaranteeing fairness between SSs. The PF scheme maximizes the entire throughput while guaranteeing some level of fairness between SSs, and exhibits superior performance. Therefore, the PF scheme is popularly used in the OFDMA communication system. In addition, the PF scheme can be implemented at lower complexity in a communication system using a single carrier (single-carrier communication system).
However, when used in multicarrier communication systems such as the OFDM communication system and the OFDMA communication system, the PF scheme increases in terms of complexity, and there is no proposed solution for the increase in complexity. The PF scheme's advantage of maximizing the entire throughput while guaranteeing fairness between SSs is acquired only when each of the SSs feeds back CQIs for all of the subcarriers. That is, when each of the SSs feeds back CQIs for only particular subcarriers instead of all of the subcarriers, the PF scheme's advantage of maximizing the entire throughput while guaranteeing fairness between the SSs is not guaranteed.